Plain English with Derek Thompson - An Astrophysicist Explains the "Strongest Evidence Yet" of Alien Life

Episode Date: April 22, 2025

Last week, a team of astrophysicists from the University of Cambridge announced that they had discovered the “strongest indication” ever of extraterrestrial life. The source did not come from Mars or Venus or any nearby moon. It came from K2-18b, a massive planet some 120 light-years from Earth. If this finding checks out, it is, without question, one of the most important discoveries in the history of science. But many scientists think that ... well, it might not check out at all. Today’s guest is Sara Seager, a celebrated astrophysicist at MIT. Seager is a pioneer in the study of exoplanets and their atmospheres. She has done as much as practically anybody to develop the science of interpreting light from faraway stars to make inferences about planets. In today’s show, Seager and I slowly worked our way up to last week’s announcement by building a foundation of the basic science at play. What are exoplanets? How do we know that they’re there? How do we have any idea about the chemicals present on that planet if we can’t send probes to test their air? What does the K2-18b finding really tell us? And what larger philosophical questions about life and aliens are raised by this new science of exoplanet atmospheres? If you have questions, observations, or ideas for future episodes, email us at [e-mail]. Host: Derek Thompson Guest: Sara Seager Producer: Devon Baroldi Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Royal Port Rush, away we go. Today, the search for alien life. Last week, a team of scientists from Cambridge announced that they had discovered what they called the strongest indication ever, of extraterrestrial life. The source did not come from Mars or Venus or some nearby moon. It came from K218B, a massive planet some 120 light years from Earth. By analyzing light transmitted to the James Webb Telescope from a faraway star,

these scientists claimed to have detected hints of two special molecules, dimethyl sulfide and dimethyl disulfide. and dimethyl disulfide. As far as we know, these molecules are only produced in abundance by water-based life, like algae. In fact, these molecules are partly responsible for that salty, sulfurous smell

that we sometimes associate with the sea. The paper's author said that this finding strongly indicates what they call a Heishian planet, with an atmosphere made of hydrogen gas, hovering above a large, planet-wide ocean, a water world, teeming with life that's coughing up special molecules giving away their presence. Hycheon here being a portmanteau of hydrogen and ocean.

Just imagining what this planet would look like makes me so excited. If you saw the heartbreaking scene in Christopher Nolan's movie Interstellar, you can imagine crash landing on a water world. But from what we know, from what lives, we know, K218B would probably look very different. The planet orbits a red dwarf star. So if you stood on the deck of a research submarine, bobbing in this planet's endless ocean, the sky might be filled with towering anvil clouds whose underbellies glowed orange-red. Every day would be an eerie golden hour over an eternal dark sea. If this finding checks out, it is without question one of the most important discoveries in the history of science.

For thousands of years, we have imagined the meaning of the lights in the night sky. Our ancestors painted the stars onto rocks and parchment and the walls of their caves. We've looked up to the stars and guessed at their meaning. In a thousand different ways, from the ancient Egyptians to Fermi and his famous paradox, we found different ways of asking the ultimate existential question. Are we alone in this universe? Last week's discovery, if true, would mark the end of asking. It would be an answer.

We are not alone. Life exists. And we have discovered its signature in the gases emitted by aquatic life some hundreds of light years away. I want to sustain your wonder and enthusiasm a bit longer here. Here is the lead author of that Cambridge paper, Niku Marasuddin, presenting findings to the Royal Philosophical Society this past November. You can practically hear the giddiness in his voice as he presents what he considers the strongest ever evidence of life outside of Earth. Let me break it to you that this is the first time humanity has ever discovered carbon-based molecules

in a habitable zone exoplanet. When you see a spectrum like that, it may not mean very much, but this is one of the most profound observations that have been made in my view in exoplanet science so far. Now, as they say, big if true,

and here's the rub. This finding, it might not be entirely true. In fact, the community of astrophysicists has major doubts about Madhusuddin's findings. A European astronomer called it irresponsible nonsense. So is this the most important discovery

in the history of space science? Or is it nonsense? Today's guest is Sarah Seeger, a celebrated astrophysicist at MIT. Seeger is a pioneer in the study of faraway planets and our work to uncover the secrets of their atmospheres. She's done as much as practically anybody to develop a science of interpreting light from faraway stars

and make inferences about the planets passing in front of them. And the gentleman at the center of this K218B news, Niku Matasuddin, she knows him well. He was, in fact, her first doctoral student. In today's show, Seeger and I slowly work our way up to last week's announcements by building a foundation of the basic science here. What are exoplanets? How do we know they're there? How do we have any idea about the chemicals present on their planet if we can't actually send probes to test their air or even see the planets directly with a telescope?

What does the K2-18B finding really tell us? and what larger philosophical questions about life and aliens are raised by this new science of exoplanet atmospheres? I'm Derek Thompson. This is plain English. Professor Sarah Seeger, welcome with the show. Thanks for having me today. I am so thrilled you agreed to do this.

When I saw the news about K218B, I texted my very good friend Ross Anderson at the Atlantic, and I said, who is the single best person I could get to come talk to me about exoplanet atmospheric science? And he said without pausing Sarah Seeger. So this is a huge treat for me. And I want to work up to the news of this moment and your reaction to it because there's a lot of science here that I need to understand to appreciate what is being claimed. I think we should start with you and your life.

When did you realize that this was going to be your career? Stars and planets and aliens, the cosmos. When did you catch this particular bug? Well, I like to think the defining moment was when I was 16 years old. And because I had to cross the university campus on my walk to high school every morning, I saw sign for an open house. I decided to go and check out the astronomy department. So I go to the astronomy department. It's like in this really tall building.

That's all physics and like the top couple floors are astronomy. I got out of the elevator. I see these like this table with students and professors and pamphlets. And it was sort of like dawn on me that you could actually be a scientist, specifically an astronomer for a job. And so, you know, the night sky had always been something I love, but I had never understood that you could take something that was intriguing and, well, partly romantic and just like so, wow, the sky is so big and turn that into a career.

I really had no idea. What are exoplanets? And how do we know they're out there? Well, exoplanets are planets outside of our solar system. Usually we like to define them as a planet orbiting a star other than the sun. Because all those stars are suns out there. And if our sun has planets, Mercury, Venus, Earth, Mars, etc., it makes sense that other stars, you know, these other suns have planets also and they do.

And how do we know that those exoplanets exist? Can we see them directly with telescopes, or are we somehow detecting them indirectly? Both, actually. I mean, honestly, we have many techniques to find exoplanets. The most productive ones so far are looking at them indirectly. So, for example, the most popular one today is we'll see a planet that is orbiting in a lucky way. Like it's orbiting just so that it goes in front of the star, as seen from our viewpoint, over and over again, every orbit. And while we don't spatially resolve the star, we only see the star as a point of light. We can monitor the brightness of that star.

And you know, it'll dim by the tiniest amount when the planet goes in front of the star. And when the planet is finished going in front of the star, the star returns to its normal brightness. There is a lot packed into that answer that I want to slow down for. You mentioned that we don't see many exoplanets directly. We pick up their presence indirectly. and I want to get to that in a second. But first, I've read that the science of detecting exoplanets is very, very new, like barely 30 years old. In the 1990s, when you were starting your career, many people were skeptical that we could discover exoplanets.

Why? Without question, everyone understood that planets exist. Because we never just find one thing in science, like the fact that our sun has planets. But the evidence that there should be other planets out there were stars that are being born. They are all, without question, accompanied by a disc of leftover garbage, if you will, like dust and gas, and we saw those for a very long time, and they have to be forming planets. You know, it's like you have to have a dust bunny under your couch at some point in your life. You know, it wants to form.

They just want to form. So it wasn't that people doubted there were other planets around other stars. It was just that the level of number of decimal places you would have to go, like the precision of the measurement you'd have to make to find them, just seemed laughable. Okay. So that's why it was so crazy. And then people were searching for solar system copies.

In science, it's never good when you only have one example. Like we had our solar system, we've known about that for millennia. But wow, searching for Jupiter. it is far from our star, it is very hard to find. It has an incredibly weak signal. So the reason for all the skepticism was when the first exoplanets were found, they were found orbiting sun-like stars, but they were found, they were the easy ones to find. Big planets very close to their star, Jupiter mass planets, many times closer to their star, than Mercury is to our sun. And remember when I told you, we've seen disks of gas and dusk

that surely are forming planets? Well, that close to the star, there's not a enough material to form a massive planet. So people couldn't accept it. They couldn't accept these planets for real and tried to explain it away with other phenomena having to do with the start itself. So there's definitely a lot of skepticism early on. And that skepticism continued. I mean, first you had people saying, well, we can't detect them. Then you had people saying, well, we've detected something. It's probably not a planet. Then you had people saying, okay, fine, we've detected planets, but it's going to peter out as we find the easy ones and dead end before we get to the harder ones to find.

Is there a certain type of exoplanet that is most extreme, that it would seem most alien to earthlings? Like, if we managed to get like a rolling camera shot of several types of exoplanets, like what planet type would be the most strikingly bizarre or sublime? If you could like set up a GoPro there and just capture hours and hours of footage, provided, of course,

the GoPro didn't like melt immediately upon contact with high temperatures or sulfuric acid or whatever. But like how crazy do we think some of these exoplanets are compared to the familiar neighborhood of solar system planets that we have here and you know near near to planet Earth? Honestly, they're probably more crazy than we could possibly imagine. But so far, believe it or not, we don't know if all the planets out there will fall into neat categories. Let's say we fast forward 20 or 30 years and we have, hey, there's 30 categories of planets? Or is there truly a continuum where there are literally thousands of different kinds of planet types we don't know? And the other point I want to get across before answering your question is, it's probably the

opposite that they're all crazy and exotic, I mean, compared to our planet, because you know what's amazing is that we find exoplanets in all masses, all sizes, all orbits within the laws of physics. There's literally a continuum of planet types. It's like, you know, we have adults and we're all a certain size, and there's a distribution, and maybe there's definitely outliers. But it seems more extreme than that. You know, there's not like this average.

There's just a continuum of options. But I could still pick a couple for you. I really like their planets so close to the star. There are some, we call them, super Earths. They're planets about Earth's size or larger. and they're so close to their star. Based on Coupler's third law, they also orbit the star very quickly.

So their year, the time it takes to go around the star, is less than half of an Earth day. Yeah, and these planets are so hot, we think their surfaces are melted. So you'd have like lakes or oceans of liquid rock, not from volcanoes, but just because, just because of the heat from the star. So we're talking about waves,

tsunami waves of lava just churning over a planet that might itself have very little rock itself. It's just massive oceans of lava with one-day revolutions around the sun. Maybe. We really don't know. They're pretty crazy. There are other planets that are extremely mysterious. The planets that are larger than Earth, they're about two to three times the size of Earth, but smaller than Neptune, where Neptune is four times Earth's size. And these planets defy all explanation. We don't have a solar system counterpart, yet. They appear to be the most common planets in our galaxy.

And why are they so mysterious? Because let me just give you a quick thought exercise. Let's imagine I'd give you a box. I arrive at your home or your apartment, and I'm like, hey, here's a gift. And I tell you, you can't look inside, but you can guess what it's made of. And there are three options, planetary materials. It could be rock and iron. or it could be water or it could be hydrogen.

Now, if I give you that box and, oh, it drops to the floor, it's just so heavy. You could make a pretty good guess as to what's in the box. Yeah, it would be some rock. Or if I gave you the box and you just let go for a second and it floats away, I think you could probably guess what's there. Hydrogen, it's a very, like when you had the helium balloon as a kid and you know, you accidentally let go and then cried because, yeah, it floated away. Well, if I gave you a box, it just seems so average.

Like, it's not too heavy, not too light, it just seems blah. Well, you can't tell what's inside. It could be water. It could be rock with hydrogen. It could be rock water and hydrogen and some mixture. And that's the problem we're facing for this intermediate, you know, size and mass type of planet. That's so common. They're just this intermediate average, and we aren't sure what they're made of.

That's actually one of what K2-18B falls into this. And we're just kind of scratching our heads and arguing about what it's actually made of on the inside. So think about that for a moment. The most common type of planet we can find so far, we don't have a solar system counterpart. It's just because of maybe bad luck, it's, you know, average mass and size and average density makes it hard to say what it is until we can observe the atmosphere. We're hoping that will sort through it. But so far, it hasn't, you know, separated out what we really need it to. Your work has been groundbreaking, not only on the detection of these exoplanets we've been discussing, but also on determining what's in their atmospheres. If these exoplanets are often just blurs on telescopic imaging, how can we speak to what their atmospheres are like? Well, every gas has a special fingerprint.

That's like we might, you know how every, it's not quite as good as this, but just for an analogy. supposedly how every human has a unique set of fingerprints. But if we just got like a little slice of one of your fingerprints, it would be a lot harder than if we had all five, you know, of your fingers and thumb. And so in the ideal world, if we had as much information as possible, so we could see the atmosphere, spread out the light into its constituent colors very finely, and see all the different gases in the atmosphere, we could try to piece together what's there. But it ends up being much more complex because you use the word blurry, and it's blurry in the sense of we don't have very fine wavelength resolution. We don't see all wavelengths. So it would be just like seeing the part of someone's finger rather than every finger. So we do have that. That's in our favor, right? That every gas absorbs in a slightly different pattern at slightly different wavelengths. That's kind of the fundament of everything.

This work is so magical to me that I really want you to explain it at a deeper level of specificity. When I was researching for this episode, I kept coming up on this word spectroscopy, as the science that we're running on the light that's being transmitted from faraway stars that we're using to determine the atmosphere of faraway planets. What is spectroscopy? We've had this field of science called spectroscopy. for many, many decades, if not a couple of centuries. You know, in high school, you do the Bunsen burner thing, and you put sodium in,

I hope everyone remembers this, and you use, like, a slide, and you see two lines like brightly glow. So, like, as a society, we've been working with this for a very long time, with stars and galaxies, and even just, like, the surroundings here on Earth. So we didn't invent, like, everything from scratch, but we're taking these, you know, old ideas. we're using Earth-atmospheric science, solar system planet science, and now kind of crafting it. Crafting is the right word here

in a way that works for exoplanets. Yeah, I watched a presentation that Niku Matasudan did for the Royal Philosophical Society this past November, and he was showing the audience what you were describing, an atmospheric spectroscopy readout of an exoplanet. And it looked to me like light wavelength, were along the x-axis.

It was a plot. It was between it with an x-axis and a y-axis. So light wavelengths were along the x-axis, and then some measure of their presence was along the y-axis. It seems like he was suggesting we can somehow estimate the chemical makeup of a planet's atmosphere

by studying what kind of light travels through that planet's atmosphere to our telescope, right? And so we're doing like a, a study of that light to determine what chemicals are present in the atmosphere that's being reflected to us. Transmitted. Transmitted.

Transmitted. Through the star. Through the star's rays go through the planet atmosphere. And some of those rays get blocked depending on what's in the atmosphere. And so what's transmitted through and what's not transmitted through, we piece it all together. Correct. This is just amazing, right? Do you ever step back and think, we are these little tiny mammals on a planet?

And we built this machine that can capture the light from stars hundreds of light years away. And whenever a faraway planet interferes with the faintest twinkle of starlight, we can run an analysis of that little moment of interference and say, oh, it's atmosphere must have carbon dioxide. Oh, that atmosphere must have ammonia. And from that, we can guess what's beneath the atmosphere. I mean, it's something close to magic. It is amazing. And I'm super proud of this myself

because back in the last year of the last century, 1999, my very first project out of my PhD was describing this method, introducing this method. So now that I can reflect back for a moment, I mean, wow, there's just so many planets out there. Dozens to hundreds are being observed right now. And it is amazing. I mean, we're here fighting over details and arguing,

but you're right, like stepping back, it's just phenomenal. Okay, so flipping from the wonder to skepticism, when we get these readouts of the atmospheres of faraway planets, is this like a very clear photograph where everybody agrees what's in the photo or is it something very, very fuzzy where there's lots of room for debate? Like someone looks at the readout and says, oh, planet Derek has a bunch of carbon dioxide and somebody else runs their own analysis and says, no, that's clearly hydrogen gas. Like how much is up for interpretation here? You were already alluding to this, that how do we tell what's there?

Well, the thing is, right now, there's not enough information to have a unique interpretation. So, for example, we might detect water vapor on a hot, giant, puffy planet. But we might not be able to tell you exactly how much water vapor is there. And we might be able to say, hey, I see carbon dioxide. I'm like 95% sure it's carbon dioxide, but it could be a lot of carbon monoxide instead. So we have this kind of gray kind of shady way of interpreting. We have a name for it, actually. My Madhue was actually my first PhD student way back at MIT when I was first a professor there.

And together, he and I literally, I had hesitate to say invented, but we put together this technique we called and is called Atmospheric retrieval. It kind of sounds like, you know, you're going to your refrigerator and you're retrieving some milk. But you're kind of trying to retrieve parameters, how much water, how much carbon dioxide, how much of this, what are the temperature ranges possible? So instead of us getting the data and saying, hey, here's exactly what the atmosphere is, here's exactly what's there, instead we map out like the range of possibilities of the gases and temperatures. Does that make sense or you could perhaps, you know, translate it. So in a weird way, the analogy that occurred to me when I was reading about this science, this ability to translate wavelengths into pictures of an underlying

reality is something like, imagine if somebody gave you a very short musical recording of a choir, you could hear just a few seconds of vocal harmonies in this little recording. You could listen closely. You can even run an audio analysis to isolate the number of people singing the various parts. So let's say you determine that they're singing a C major chord and you detect what sounds like 10 bass singers singing the bottom C, five tenors on the E, one or two sopranos on the high G note. So you can't see the choir itself. But essentially by studying the volume of different vocal frequencies, you can make an educated guess of how many men and women were in the room. This is kind of like what you're doing, but you're doing it for light rather than for sound.

You're running an analysis of the components of the light to determine the character of the atmospheric interference. I love it. I love this analogy. Yes, we're definitely doing this. I love this. And let's make it a little harder now. Let's make it a little, maybe the recording's not so great.

maybe, you know, it's not a great recording, and there's a bit of uncertainty in what you're hearing. And now you are doing one, I'm doing another one, and like five other people are doing their own. And we get slightly different answers, unfortunately. Interesting. So I'm like, I'm hearing like a lot of base notes, clearly there's more men in this choir. And you say, actually I'm hearing it's a little bit more of like a blended melody.

Maybe there's more women in the choir. And so there's like a little bit of an interpretive tug of war over what we're actually hearing from this recording, even if we're all working off of the same MP3. Right, and sometimes, let's say we have large agreement that I agree, yes, there's more bass, there's more men, there's more this, there's more that. But you help me out with the analogy now,

but we still have another step of interpretation. So the first step is we get the recording, maybe it's a bit messy. Then we interpret how many bases, how many this, how many that. But there's still one more step. We have to perhaps classify this song. Is this an area? Is this like a takeoff?

on jazz. I mean, help me out here. Like, we have to help me out with something that takes the one more step we need to make it really analogous with exoplanet atmosphere interpretation. We would be trying to determine if they were singing a Gregorian chants

or if they were singing a Benjamin Britton song or if they were singing a Taylor Swift song. And so we're trying, I mean, there's that sort of genre that you could do. No, I like it. I like it. I like it. But it's also like, it's almost like determining from the fact that we can hear more base in this recording, we can assume that there weren't women in the room, right?

It's like making further determinations of like, maybe like the politics of the room. Like this is not a space that allows women because we hear so many, so many like men in the recording. And so we're making sort of second degree interpretations. Yeah, second degree, right, right. And you can see how sometimes, just like our first analogy of the box I brought you,

if you're an extreme corner parameter space, we call it. And everyone probably is going to agree, right? If it was like all base and not a single other thing, that might be something we could all agree to the first and second degree of interpretation. But you could see how, or perhaps if they were all sopranos, but there's some middle ground, right, where it's just tricky. And we have to really kind of push, not very scientific. We kind of have to push our analysis with a lot of judgment, subjective judgment. Okay, so this analogy works for me because it helps me,

really visualize how we are turning a starlight transmission into a readout of faraway atmospheres. But let's make sure we hold on to the practical aspect of this science. Why do we care so much about what kind of chemicals we are detecting in the atmosphere of exoplanets hundreds of light years away? Well, we're hoping it sheds light on what the interior of the planet is made of and answers your first question of what's out there. these planets. We have masses and sizes and average densities. We don't know much else about them. And so why is it so important? Because we're hoping it's going to connect us to the inside and

finally know what the planets are. You know, especially this middle ground planet we've been talking about the so-called sub-Neptune-sized planets. That's one reason. The other reason is we all want to find a sign of life. And the sign of life, the way we're looking for it, is a gas in the atmosphere that doesn't belong. That's way out of equilibrium with the other atmosphere. constituents. These are sometimes called biosignatures. What are examples of gases that would be biosignatures versus types of gases where if we found them in abundance on faraway exoplanets, we'd think nothing more of that exoplanet in terms of a possible source for life? Well, our favorite biosignature gas is the one right here at home, oxygen. We have molecular

oxygen in our atmosphere that fills our atmosphere to 20% by volume. But without life, without plants and photosynthetic bacteria, we would have basically no oxygen. So if there's one crazy factoid for you, it's that our entire atmosphere has been re-engineered by life. Now, what's so cool is that nearly 100 years ago, an astronomer wrote about oxygen in our planet atmosphere and realized it could be a sign of life on another planet. And said astronomer actually took observations of Venus, looking for oxygen and Venus's atmosphere, and didn't find it to some limit

and put an upper limit on the amount of oxygen in the Venus atmosphere. We're going to get to Venus in just a second because you've done so much interesting work on Venus. But just to ground us here, what makes this moment right now so exciting for exoplanet science. What's so amazing now is that we, like me, you, everyone,

we're the first generation in human history that has the capability, namely the James Webb Space Telescope to start with, to now try this out on exoplanets, to look at their atmospheres, to look for gases, that might be a sign of life. And do you know what the most crazy thing is right now?

That now any gas someone comes up with, including oxygen. Another team will find like a counterfeit, argument against it being a sign of life. So we're in this weird kind of back and forth. I almost feel it's like that day you decide to clean out your entire closet. You take everything out and then it's a giant mess. It's going to be a while before you put it all back, sort through it all.

So I feel like because it's upon us now, we're just finally unpacking it all and realizing that even oxygen might be able to be made in large amounts without life. So I take this to mean that we are trying to produce a kind of glossary of biosignatures, where if we detect these chemicals in our spectroscopy of exoplanet atmospheres, we might be able to say this could be an indication of life. I love that. I need to get you as my PR person, because do you see all those caveats you put? No one likes that. Nobody likes it.

And most journalists and podcasters, they can't get there yet. But yes. Doing my best. Can you just go one level deeper on oxygen? Like, I think I heard you say on another podcast that in a way, let's say an alien civilization was producing this exact same science on planet Earth. And they run their spectroscopy and they realize that planet Earth has an enormous amount of oxygen. I think what you said is some people,

Some alien scientists in that room might say, there's no way there's life there. They must have fires all the time. Like, their entire planet must be just consumed by fire because oxygen is so flammable. Can you talk maybe a little bit using that as an example of how maybe, like, some biosignatures might be both positive for signs of life and also negative for signs of life? Right, right. It's not my original idea, but we love to imagine an alien civilization on a planet orbiting a nearby star with the kind of telescopes we're trying to build now, looking back at Earth.

And we should say, think about that. Oxygen is global. It is so visible. So it's not that they'll think life's here because of big things like the Great Wall of China or city lights are pollution, but it's oxygen. And then you sort of think about a bit more. And well, first, they're probably fighting over data analysis. And next, you know, there's this sort of view that maybe they know Earth is here

and they look at our atmosphere and our ocean. And they're like, nah, water is terrible for life. It is actually, I started working in astrobiology, and it's like sometimes to get stuff to work, you need a very special conditions in the water. They'd say water, it's, you know, too destructive. They'll look at oxygen and be like, that much oxygen, they must have large regional fires at any time. How good life survive? And so, you know, there's always two sides to the same story. And that's serving to be true now for most gases we can think of.

So we want a menu of options. I actually put a list, I wrote a review paper recently. It's on the archive. And it's not formally published yet. It's impressed. But we actually did what you wanted. We gave a menu. And we have three columns and check marks and crosses like a scorecard.

So what we don't, there's not, you know, everyone has their own opinion. But I don't think we've converged to, hey, here are the top five gases. If we see them, we'll believe it's life. I had to just recircle this one point. you mentioned that these alien scientists several light years away would look at the amount of water on Earth and say that seems destructive. But right now it's my understanding that astrophysicists and astrobiologists are in fact looking for water. So it seems like even here, the presence of water is both somewhat a necessary condition for life, but also potentially an enormous destroyer of life. Is that the point that you're making?

I'm glad you picked it up because it's very subtle. The large majority do think water is the only solvent for life and that we have to find water on another planet to even assume any remote possibility for life there. I did a poor job of trying to copy one of my colleagues who will say like organic chemistry is very finicky, so finicky. And if you ever tried, like I'm learning to do some biology and biochemistry, tried to do stuff.

Like sometimes a protein will fold just right, but oops, the condition was a bit wrong. It didn't fold at all. Or, uh-oh, this stuff now just clumped together into a giant clump. It's not going to do anything useful. Like, if you just start digging in a little bit, you're like, I just speak for myself.

I'm shocked life even exists here on Earth and water because some things require very, very sensitive conditions, like pH or like it needs this buffer, it needs that little bit of salt. So some organic chemists will be, you know, If they're doing chemistry, water wouldn't be their first choice to do, not life chemistry, but any chemistry in. So that's what I was trying to get at the two different things. Our life is in water, therefore we need water. If you start drilling down and looking at specific things that happen in water, it's not super robust.

Like, it's something's very hard to coax something to happen if the conditions in water are not just right. Tell me that your work on Venus and why you're so interested in studying Venus for biosignatures. Well, I think I should share with you how I got into studying Venus because I was one of the people who for literally 15 years, maybe 20 now, have been trying to come up with the menu of options of biosignature gases. Which gases in which context are a sign of life. And my favorite gas has been phosphine. That's a phosphorus atom attached to three hydrogen atoms. Now, why is it my favorite? One of the reasons is on Earth,

phosphine is only associated with life. It is what we call thermodynamically disfavored. It's hard to make phosphine. So it's not just going to come out of a volcano or just be floating around. Like, you know how we have so much carbon dioxide? It's not like that. It's just not going to happen unless it's something

a lot of energy has to be put into it. And we were innocently working away on all the lists of gases, including phosphine. When surprisingly, across the globe in the UK, another astronomer was working on phosphine as a possible biosignature gas, Professor Jane Greaves in the UK.

And she purposely set out to find signs of life on Venus using phosphine gas. And the reason she was doing that was because phosphine has a signature, a spectral feature at radio wavelengths where she's an expert astronomer. Long story short, someone connected our two teams, because we were working on phosphine and what it meant

and the interpretation and the context, and she was working on observations. So she invited us to join her team to help with interpretation of her data to help her write more proposals. Well, in the fall of 2020, we as a team made an announcement. We reported the detection of parts per billion, tiny amounts of phosphine gas in the clouds of Venus. And just like on Earth, phosphine shouldn't be there. There's going to be. the phosphorus atom. P, it wants to attach to oxygen atoms in an environment with little hydrogen and not the right temperatures and pressures. So we also wrote 100 pages. Now I know that quantity doesn't always mean quality, but this was quantity and quality, explaining all the ways that

phosphine could be made on Venus, lightnings, lightning, meteorite, delivery, and burn-up, volcanoes. And we showed how each process, even if it could produce the tiniest amount of phosphine, couldn't produce enough phosphine to match the observations. So we put all of that out there, and do you want to guess at the reaction? Was it positive? Well, we also didn't say there's life there, but we said this leaves room for life being there.

It was like partly positive initially, but our scientific community hated this so much. They were just angry and very upset. Why were they angry? Well, I think they were angry. I can't say for sure, but I think they felt like we were being irresponsible in making this claim. Even though we had done years of work, we had worked with the radio observatory team, the technicians at the big facility, Alma, we had done what we thought was a very careful job. But it was a tiny signal in very messy, noisy data. It required a lot of data analysis, data processing. So initially they just thought there's just no possible way. And we're naturally skeptical, right? Like if someone told you, hey, I just saw Bigfoot, what would you say? See? I can see you smiling. I'd be like absolutely no fucking way. Yeah. Okay. Yeah, that's exactly the reaction people had. It was just so ludicrous. And we can get to why in a moment. But it became a bit more serious

because all the data is public. People looked at the data, analyzed it quickly, though. They didn't take the five years we did. They just boom, went through it. And some of them took a bit longer, went through it, and the first problem is that many teams did not recover the signal in the data. Some teams did recover the signal, but then moving on to a second criteria, they wanted to associate it not with phosphine, but with a different gas, sulfur dioxide, which is already present. Let's assume for a moment that we, just for the argument sake, believe that, yes, the signal's there, number one, and number two, we also believe that it's associated with the right gas, phosphine. then there's still, number three, is it made by life or is it made by something else?

Now, there's unknowns and unknown unknowns and unknown unknown unknowns. Like, we just don't know. There's a lot that could be going on, and people, you know, constantly, basically attacked on all those three levels. So this gave me a problem. It wasn't, it wasn't the very bruising experience that phosphine on Venus was or is. It remains controversial. But it was, wow, I've set my career on finding signs of life on an exoplanet. And we will have to deal with those three criteria. Is the signal real? Is the gas, if the signal is real, is it attributed to the right molecule? And number three, if those two are correct, which I have no doubt we can do number one and two eventually.

Number three, is it produced by life or an unknown process? Now let's circle back to the beginning of our conversation. asking me, what are these like? Which ones are exotic? If we could take a film that, you know, pan a video, I'm like, we don't know. We just don't have enough information on what they're like. So how are we going to handle number three? Is it made by life? So I had, I had to like just stop. You know, it's like you need a timeout if you're like a kid and you're like, not melting down. It's the wrong word, but, you know, you get, I had to do like a self timeout from exoplanets. And naturally, it seemed like I should look at Venus. It's, wow, Venus is,

so interesting. And what phosphine did for Venus was shine a new light on Venus. It gave it attention that it deserves. I always liken it to the siblings. Like, I don't know if you have siblings or you know siblings. You have a sibling? One sibling. Yeah, younger sister. Okay. So there's always the one sibling that gets all the attention. And there's one sibling that's always ignored. Well, Venus is the ignored sibling. Like, do you ever even hear a Venus? No, you don't hear about Venus. You hear about Mars. Mars is that sibling. So I don't know which one you were. was like more like Venus personally, but. Why are we looking into the clouds of Venus rather than the surface?

Well, the surface of Venus is too hot, way too hot for life. It is 700 degrees Kelvin. It is hot enough to melt lead. And Venus, you know, has this massive carbon dioxide greenhouse atmosphere making that surface just on just yet not not suitable. But just like on Earth, if you go on an airplane high up or you hike up a mountain, it gets colder and colder above the surface. And that happens on Venus as well. And so 50 kilometers above the surface of Venus, it's actually the right temperature for life. In fact, oddly enough, it's the same temperature and pressures right here on Earth. And way up there, there are clouds. On Earth, we have life in our clouds, bacteria that they don't maybe want to be there. They're getting swept up. They go inside the

clouds for a while. They get rained out. But on Venus, unlike on Earth, the clouds are, they're not fragmented. They're always there. It's always cloudy on Venus, everywhere on Venus, and the clouds are 20 kilometers thick. So it sounds kind of good. I've sold it like as, wow, sure, it's a good place. There's liquid. There's the right temperature and pressure. But these clouds are not made of water. They're made of acid, concentrated sulfuric acid. So if you say there's life in the Venus clouds, it's like saying the Bigfoot, right? There's life in this acid that destroys all of our life. we use it to clean electronics, we use it to kill things. It just seems so crazy.

And so life would have had to evolve, or maybe just be created, in these clouds, and is sort of maintaining its sort of bacterial microbial presence in clouds made of an acid that on earth we use to kill organisms rather than sustain them. So it's possible, but it would, it requires the existence of an alien life form that is very literally alien to everything that we understand to be life. Well, that's what people would have thought, and I love exactly how you said it, because that that was basically what everyone assumed had to be true. And it's still true that none of our life survives. Our DNA doesn't survive. But what my team did is we went to the laboratory to use

sulfuric acid, and we started to put biomolecules in sulfuric acid to sort through is just everything unstable or only certain parts of certain molecules unstable. And we found some astonishing, like, shocking things. We put our 20 biogenic amino acids in sulfuric acid, and we found, with one exception, they're all stable. Some are chemically modified, but they're stable for months, like the months that we studied them. Although our DNA is unstable,

we found that the latter part of the DNA, the nucleic acid basis, ACGT, when you see the little picture, it's the latter, stable. stable for years even. Well, we measured it for a couple of years. It's probably still stable. It's interesting because I had read that many people looking for exoplanets that were amenable to life, were looking for planets that had solid ground where life could evolve and not be overwhelmed

by water or whatever else. But already here, we're talking about the possibility of life in the clouds. And now there's this moment's big new story, which is life. in water. So I want to turn to that news at the moment. Last week, along with several million other people, I suppose, I got this push alert from the New York Times telling me that a possible signature of life, biosignature, was reportedly discovered on a planet called K218B about 120 light years away. Professor Madhusadhan and his team claimed to have detected the biosignature of dimethyl sulfide in their analysis of the planet's atmosphere.

and dimethyl sulfide was, according to Maru, precisely what you were talking about with phosphine, a chemical that is difficult to produce biotically and therefore might be a signature of some underlying life. That's my high-level summary of what I understand to have been found. What would you say this discovery actually discovered?

And what was your reaction to it? definitely tell you, well, there's a few things here to unpack. One is I hate to be on the other side. Remember, I was part of the team that reported phosphine. We said it leaves room for life in the clouds. And I dislike being on the other side of now having to react to this, the way people reacted to my announcement. But, hey, we can go back to those three criteria and we can walk through those together. So number one is the signal real. And if you read the paper and even the press release, the team said that they need more data. They said they've reached a lot. that doesn't meet the scientific standard of robustness. So that's coming from the team,

not from me. So, okay, is the signal real? Let's, there's an indication it's real, but let's wait and see. So that's number one. Number two, is it attributed to the right gas? Well, you know, the same planet K218B, a while back with Hubble Space Telescope, people announced that had water, water vapor in the atmosphere, and it was a big deal. But with the James Webb Space telescope with better data, more fingerprints, if you will, people decided it has no water vapor in the atmosphere, and it has methane instead. That's like a pretty big thing to go from, I have water to I have no water. So is it a attribution to the right gas? Not sure yet. The paper, they went through different gases, but there's more to work to do there. So number one didn't check yet, number two

didn't check yet, but presumably we can get more data, we can do more work, we can deal with those later. Number three. It's true on Earth that we only see DMS and large quantities by life. That is definitely correct. But we have yet to see in a hydrogen-dominated environment, you know, this molecule has hydrogens on it, and presumably this planet has sulfur. We haven't seen yet. People haven't fully explored all the ways one might make a lot of it. So with everything there, we just have to say the jury is out. That means we don't know yet. Do you have any feelings about the Heishian thesis that your former student, Madhu, has been advancing. This idea that we've historically looked for a relatively narrow band of sort of habitable

planets or planets that are amenable to life. But there's a possibility that there's a new kind of planet, hydrogen-rich atmospheres and almost entirely ocean-based underneath those atmospheres that could expand the range of planets that we consider possible host to life and say maybe this whole sub-Neptune category that you, Sarah, we're talking about, maybe it's a bunch of water worlds. And much of what we think of as life in the universe is actually profoundly water-based. And it's Earth that is weird with all the land that we have. And what's more common is just a bunch of microbes and algae or something. You're shaking your head. So I might have gotten something in the question wrong. But how to

No, well, there's several things. First, I love the idea because my second student and I put that idea out that these so-called mini-Neptunes, they could be a variety of things, they could be water worlds. And water worlds came even before that from some other people. But yes, we always want to expand our definition of what type of planet could host life, because we have so limited options to get data on. So we definitely want to do that. I was just taking my head because everyone has their own opinion on stuff.

I don't know if I subscribe to this opinion. But the reason why people want land is they think like oceans are too diluted and that you need land because you get runoff from the land and it concentrates like minerals and metals and like stuff life actually needs. People think that on land you can concentrate molecules by having like little pools that, you know how when like water evaporates then you've got little like spots left over?

Those are good. That that's how you can concentrate materials. So a lot of people dislike like the whole ocean. without land because you've no way to concentrate materials for life to form. You've no way to concentrate nutrients. But I'm not sure. I like to keep an open mind. So I definitely like the idea of anything. Let's look everywhere we can. It's probably not going to be, oh, just water. It could be sulfuric acid water, probably everything. Honestly, life probably goes wherever it can. So I'm definitely in a favor of pursuing this.

If I really wanted to understand, like what's going down in the astrophysicists group chats, right now with K218B, is the skepticism mostly about exactly what gas is being indicated by the spectroscopy, or is it something else? What's really interesting about K218B is different groups have landed on different interpretations of the very nature of the planet. It's just like our analogy, did we land on Taylor Swift or did we land on... Gregorian Chance. Gregorian Chance, or did we land on?

And so there's this one intriguing thing about K218B, because if it has hydrogen and methane, it should have other hydrogen species in the atmosphere like ammonia. But here's the thing. Ammonia is so water-soluble. If you have an ocean, probably all that ammonia dissolved in the ocean and there's no ammonia in the atmosphere at all. And that's one of the reasons to favor this water world, the hot ocean hypothesis. But it turns out that many of us astronomers didn't quite appreciate this, unfortunately. Well, we have the geotypes. And they said, you know what? Nitrogen species also dissolve in hot liquid rock. Another interpretation is you have this hydrogen envelope. It's a very strong greenhouse. And there's no water, but you have like a magma ocean. You have liquid rock there. And that's where the nitrogen ended up going. And there's no ammonia in the atmosphere for that reason. So there's two extreme interpretations. There's a few other interpretations as well. And we're still living with that. Admit the report of a biosing your guess. I like that we're being cautious and skeptical about exoplanets here, but do you think it's possible that in the next, say, decade, astrophysicists will make a discovery that is universally recognized as the clear finding of alien life. Is that universal eureka moment coming in the next decade?

Well, I don't think it is, but let's start with the good news that the fact that we're here talking about it is just phenomenal. The fact that we have, you know, real data, we're arguing about it. It's premature, but we're all, wow, talking about it. That's just amazing. If we go back to my list of criteria, it's number three, that would keep me up at night. What is compelling evidence for you, you know? Is it models in the computer? Is it some better, you know, what is going to convince us? What's going to convince you that there isn't some chemistry we hadn't thought of or chemistry someone's going to think of two years from now and that it is indeed life. I think that's where we're stuck right now. I think it's more likely to be a gradual thing. It's not going to be one

day, but just like with exoplanets, I don't think there was a defining moment where we're like, okay, they're definitely here. It was this sort of gradual build of evidence until today's children grow up in a world, just, of course, there are exoplanets. We've talked about biosignatures. Is there something like techno signatures that we could look for. Like theoretically, if there is extraterrestrial intelligence or superintelligence, we might be able to see something much bigger or clearer than a possible signature of methane in the atmosphere. We might see enormous structures, structures at the size of a Dyson sphere or something. Is this being talked about or looked for?

Or is there really no reason to develop technology for looking for things that are all already so big and obvious that we would find them without some kind of advanced science. Well, I'd say it's all the above. It's still a bit fringe to go after techno signatures, but we definitely support techno signatures. And most of the things you're talking about, there would be so big they should be in our data anyway. So there are groups pursuing this, getting archive data, getting a small amount of money to pursue archive data to search for signals that might be there. And so I'd say, yes, it's definitely worth exploring. and being explored.

If exoplanet atmospheric science is one major leg of our attempt to find alien life, planets that can support life light years away, what are the other legs of that stool that are most mainstream that we're using in order to detect the possibility of alien life?

Another leg is certainly our solar system, much closer to home, including my favorite Venus atmosphere, not mainstream, Mars, subsurface of Mars, the plumes of Saturn's moon and celadus, perhaps Jupiter's moon Europa. Titan has liquid, liquid ethane and methane, like liquid gasoline lakes.

So being able to go to one of these planets find very complex molecules, like themselves, not just gases, but actually analyzing large complex organic molecules, and eventually bringing a sample back here to Earth. I will say that even if the discovery from last week is admittedly premature, one thing that I found dreamy about it was this idea of a planet populated entirely by aquatic species, maybe even quite small aquatic species that we wouldn't even recognize as being life. It introduces

the possibility that while we're looking in many cases for life as we know, know it, there is actually an enormous amount of life as we don't know it out there. How do you factor that into your philosophy and your science? Well, we definitely try to ignore that, and we just claim, since we're not biologists, we're not responsible for what life is or looks like, but what life does is what we're focused on. Life metabolizes like we do, and we're assuming that life out there uses chemistry like our life does to take energy from the environment to store energy and to use energy and in the process generate a biosignature gas. So we're openly acknowledging that there could be all kinds of life out there that we could never see, whether it's that subsurface

life or deep in the ocean or whether it uses something other than chemical. Like maybe it's using mechanical energy and there's no gas. We can only look for signs of life using our astronomical tools. I will say one thing that I found incredibly inspiring about this report, because I did not go into reading this, understanding much about exoplanets or atmospheric science, is that different people look up into the sky and draw different interpretations of it. Tell different stories about it. Someone looks up into the sky and they see Greek figures and someone else looks up into the sky and they see the heavens. and someone else looks up under the sky and sees anxiety,

the fact that they're so small and the universe is so big. And it's just so inspiring, frankly, to think that we found a new way to pull meaning out of the stars. I mean, that's what your science does. It takes light and it runs it through this scientific process from which we can make determinations, debatable determinations, determinations we'd like to fight about, but determinations about what that light means.

means and what it suggests about the density of chemicals light years away, which themselves could be a reflection of life forms we can only guess at. I think it's incredibly cool to add a new interpretive lens to the sky, essentially. I don't know. I just want to end with the possibility out there in the night sky. And the next time you look up, even if you live in a city, go and look outside. And you can wonder what kind of planet is around that star. Sarah Seeger, thank you very much. Many thanks to Sarah Seeger. I just loved this interview.

I really want to hold on to two ideas from the last 45 minutes. The first is just the sheer magic of the underlying science. I mean, just think about it, a hundred light years from the Earth. This planet that we've named K218B slips in front of its star, and the James Webb Space Telescope catches the starlight filtered through that planet's

thin shell of air and different molecules in that air are absorbing or blocking different wavelengths of that starlight which means that by the time the light is transmitted to us at Earth it carries a record of the molecules

present in that planet's atmosphere and so we can study the light to determine the planet's molecular composition and even make inferences about what its underlying world is like. Is it a vast ocean planet? Is it a rocky planet? Is it a liquid magma planet?

All of this we can make educated guesses about by studying the light. And this science of studying the light is called spectroscopy or exoplanet spectroscopy, and just this idea that science is helping us paint a picture of faraway planets and possibly even, determine which ones have life. I just think this is absolutely frigging incredible.

But the deeper point that I'm left with here is that there's this toy model of science, which claims that science is about the discovery of things that are certain. Science is the discovery of certain truths. But one thing that we heard from Sarah and one theme, I think, of other interviews that we've done with really great scientists

on this show is that when you talk to responsible scientists and you really listen to the words they use to describe their work, you realize that their work is really anything but the proclamation of certainty. Science is not the discovery

of simple truths. It's more like the often weird analysis of complex uncertainty. Good science is hard, good science is responsible about dealing with uncertainties. And that takes a lot of money and a lot of time. And I just have enormous respect to the people who devote their lives and their careers to getting this right. Thank you for listening. Back to you on Friday.